This invention relates to apparatus for bringing electrical test points on planar test members and a plurality of electrical contact probes into electrical engagement for test purposes.
Printed circuit boards containing printed wiring circuits on the planar surface thereof are generally used in the electronic art. "Bare boards" is a term used to identify printed circuit boards before components such as resistors, capacitors, etc., have been mounted on the board. "Loaded boards" is a term used to refer to printed circuit boards after the components have been mounted on the printed circuit board.
It is necessary to test both bare and loaded boards to detect any electrical errors in the manufacturing process. To this end, test fixtures of various types have been used to bring a circuit board under test into electrical engagement with a plurality of electrical test probes. The electrical test probes are generally spring probes mounted in a plate and have spring loaded probe heads, all facing in a common direction. The probe heads are very closely spaced in an array and it is common to refer to such an array of spring probes as a "bed of nails". The opposite end of each spring probe from the probe head is electrically connected by wiring to an analyzer or verifier which then analyzes the circuit board under test to determine whether, for example, there is proper electrical continuity or lack of continuity between test points, whether proper components are mounted on the printed circuit board, and whether the value of the components are correct.
Various types of test fixtures have been devised for bringing test probes forming a "bed of nails" into electrical contact with test points on circuit boards. One class of test fixtures employs a pneumatically or hydraulically actuated platen to force the circuit board such that test points thereon are forced against the spring probes. Mechanical actuation means that have been used include screw jacks and combinations of cams, rollers and levers. Such devices generally require a design such that the spring biased probe heads are not fully depressed in order to avoid damage by the forces created by the actuation mechanism. This type of device also requires a platen assembly that has sufficient structural strength to prevent bowing more than a few thousandths of an inch. Likewise the probe plate in which the "bed of nails" is mounted must also be structurally rigid to prevent bowing. In the event bowing occurs in either the platen or the probe plate, or in the event the circuit board under test is not completely planar, there is a possibility that some of the probe heads will not make electrical contact with the test points on the circuit board, or if contact is made, it is possible that the probe head to circuit board pressure will be so low that a good low resistance electrical contact will not be made.
Examples of devices of the foregoing type are typified by U.S. Pat. Nos. 3,654,585, and the article entitled "Fixture Produces Uniform Contact Pressure On Boards Under Test" by Everett/Charles, Inc. that appeared in the September, 1976 issue of Electronic Packaging and Production. In the Electronic Packaging device, a pneumatically expandable resilient device similar in shape to an innertube moves a platen which in turn moves a circuit board mounted thereon into electrical engagement with probe heads.
Another class of test fixture utilizes vacuum to draw a circuit board under test and an array of probes mounted in a probe plate into electrical contact. An advantage of this type of device is its ease and speed of operation as compared with the pneumatic or hydraulically actuated press-type device. A disadvantage of this device, however, is that the forces available to draw the circuit board under test and the probes into contact are limited by the differential pressure between the vacuum and atmospheric pressure.
In order to eliminate the problems of platen and probe plate bowing, one type of vacuum fixture has been proposed and is disclosed in U.S. Pat. No. 4,108,528. In this device a door plate and a resilient diaphragm member form opposing walls of a vacuum cavity. The diaphragm member has a rigid backup plate on the opposite side of the diaphragm member from the vacuum chamber. Rigid probes having a probe head on one end extend through the diaphragm member and backup plate and the probe heads are exposed in the vacuum chamber. The circuit board under test is positioned between the door plate and probe heads and vacuum is applied in the chamber, drawing the door plate and the combination of the diaphragm member, backup plate and probe heads together on opposite sides of the circuit board under test, pressing the probe heads into the diaphragm member. This technique has the advantage that equal and opposite forces are generally aplied uniformly over both the door plate and the backup plate. However, the circuit board under test may not necessarily be of uniform thickness, or all areas of the backup plate and door plate may not be of exact uniform flatness, or there may be other mechanical tolerance variations or there may be a combination of the aforementioned. Under these conditions, not all of the spring contact probes may be forced with sufficient pressure against the circuit board under test and cases may exist where some probe heads do not make contact at all. In this regard, it is generally desired that the probe heads be forced to a stop against the backup plate. Obviously, variation in flatness of the backup plate, the door plate, or the circuit board under test may cause one of the probe heads to have insufficient pressure exerted between the circuit board and the probe head.
The force needed to reliably provide the required electrical contact between each probe head and the circuit board under test is in the order of 4 ounces. Since the maximum obtainable force using vacuum is less than 15 pounds per square inch, and more likely 8 pounds per square inch, the average number of points per square inch which may be contacted on a printed circuit board is limited by the obtainable pressure. As a result, the practical reliable number of probe heads is limited to 30 to 40 points per square inch. Situations exist where much higher densities are desired.
Other arrangements have been devised for bringing an array of probes and a circuit board for test into electrical contact. By way of example, U.S. Pat. No. 3,942,778 proposes the use of a side-by-side parallel array of cylinders, wherein one of the ends of each cylinder bears against the circuit board under test, and the opposite ends of which are in a pressure chamber. Pressure applied in the pressure chamber forces each of the cylinders against the circuit board under test. With this arrangement, it is pointed out that the cylinders will compensate for irregularities in the surface of the circuit board, such as caused by components on the board. However, this device, because of its mechanical complexity, is expensive, difficult to manufacture and service, and the force required to actuate is likely to be greater than is required in an embodiment of the present invention.
Another type of device is disclosed in U.S. Pat. No. 3,016,489. In this device, a pressure chamber has a flexible diaphragm element along one wall which forces probe assemblies against component leads on a circuit board under test. The probe assemblies are mounted and guided in a matrix aperture plate. Disadvantages of this device include its complexity and friction between the sliding surfaces of the probe assemblies and the matrix aperture plate.